Power Supply Circuits Intro, AC Line Front-Ends

The following sections describe each of the major subsystems of typical
power supplies in more detail but without specific reference to any particular
commercial design (that is left for the Chapter: Complete Ar/Kr Ion Laser
Power Supply Schematics.

Nearly all of these designs can be used as the basic blocks for a power supply
you can build. Few special parts are required and for the most part, nothing
is terribly critical.

Throughout this chapter and the next, a quasi-standard method (mine!) is used
to identify and label some of the connections between and among parts of the
power supply, Ar/Kr ion tube, and the external world. Here is a summary
of the meanings of the notations used in the sample circuit diagrams and
complete schematics:

G = AC line safety (Earth) Ground.

H = AC line Hot.

N = AC line Neutral.

DC+ = the positive output of the power supply generally prior to any
regulator or ballast resistor.

Boost = a DC voltage several times that of DC+ required by the igniter
(starting) circuits of some laser heads.

Tube+ = the actual connection to the anode terminal of the Ar/Kr ion tube.

Tube- = the actual connection to the cathode/filament terminals of the
Ar/Kr ion tube.

Note that in this document and the associated laser power supply schematics,
voltages between 110 and 120 VAC Hot to Neutral (220 to 240 VAC between
Hots on opposite sides of the line) may be shown for power in the USA and
other parts of North America. Likewise, 220 to 240 VAC may be shown for power
in Europe and elsewhere. Where some other voltage is used (such as 100 VAC in
parts of Japan), it will be ideentified explicitly.

Note: This and all other circuit diagrams in this chapter have their parts
either numbered independently to simplify discussion or, where their function
is obvious, not labeled at all. Those in the chapter:
Complete Ar/Kr Ion Laser Power Supply
Schematics have consistent designations for components in the entire
system (both commercial and "Sam's" designs).

The design described below can serve as the basic front-end to a linear or
switchmode regulator, inverter, or to a brute force power supply using only an
additional ballast resistor (for testing only, right?). It is a simple AC
line-connected AC to DC power supply.

Note: S1, S2, and S3 can be switches or relays. Logic controlled relays are
highly desirable to enforce the sequencing requirements on the Ar/Kr ion tube
power. For initial testing, manually operated switches may be used.

S1 is MAIN POWER and switches both sides of the (115 VAC single-phase)
line. A magnetic circuit breaker can be used here but it must have both
poles coupled so both sides trip from an overload on either side (or use a
single pole breaker only in the Hot line). S1 also powers the PS and head
fans.

S2 is the Preheat switch. It is activated first to bring the Ar/Kr ion
tube filament up to the proper temperature. It also slowly charges the main
filter capacitors, C1 and C2, through D5 and R5 (these part types are
non-critical - 1N4007 and 330 ohms, 10 W typical, respectively). This
eliminates serious inrush current when power is applied to the main bridge.

S3 is the Operate switch. It is activated after the required filament
preheat delay of 30 to 60 seconds. Opening S1 should also open S2 to
prevent the discharge from continuing with a cooling/cold filament.

S3 also enables the power supply fan and the igniter circuitry. (The fan
in the laser head should be powered whenever the laser is running and until
the tube has cooled down.)

The bridge rectifier, D1 to D4, can be a module or be constructed from
individual diodes. It must be rated for at least 20 A at 400 V and should
be mounted on a heat sink.

R1 is an optional inrush limiter, typically a .1 ohm, 25 W power resistor.
Where the power is sequenced using PREHEAT and then OPERATE, R1 is not
needed.

The main filter capacitors, C1 and C2, should be high quality types suitable
for high current high ripple applications. The AC (ripple) current through
these is order of the supply current - 5 to 10 A in this case. An Effective
Series Resistance (ESR) of less than .02 ohms is required to minimize power
dissipation. Multiple lower uF capacitors in parallel may be needed to
satisfy this requirement.

Note: Additional small ceramic capacitors should be placed in parallel with
C1 and C2 to bypass high frequency noise (not shown).

The dropping/filter resistor, R2 (optional, see below), needs to handle
several HUNDRED WATTS at full power. These are available. However, an
alternative is to use part of the salvaged NiChrome element from a common
space heater either on its original mounting or wound on a ceramic, mica, or
other suitable high temperature, non-flammable structure AND provided with
adequate forced air cooling (such as the fan from the blow-drier that was
its former home). Since you may need to optimize its value, an adjustable
resistor is desirable (in the case of the heater element, cut or tap to
fit!). See the section: Constructing Low Ohm
High Power Resistors.

An alternative approach is to use a high current inductor as a smoothing
choke. The advantage of this approach is that little power is dissipated
in the inductor. The problem is in obtaining a suitable part. An
inductance of 2.7 mH is required per ohm of impedance at 60 Hz. A 6 mH
inductor requires roughly 50 layers of #14 wire on an EI type laminated
steel core (I won't even mention toroids!). The core from a salvaged
microwave oven transformer should be suitable. High current inductors are
also available commercially. For example, the Stancor C2688 is rated at
10 mH, 12.5 A (and around $45, WOW!).

Using a LARGE Variac (it has to handle the entire tube current and then
some!) to power the system is an alternative since it can be set to provide a
filtered DC voltage that is equivalent to the use of the resistor. However,
without R2, the ripple will be slightly higher so there won't be quite the
full benefit of lowered DC to the regulator.

R3 and R4 are safety bleeder resistors to discharge C1 and C2 in a relatively
short time once power is removed. Using values of 10 K ohm, 5 W for R3 and
R4 will result in a time constant of about 25 seconds for C1 and C2 of
2,500 uF. In a minute, the voltage will be low enough to be safe - though
a few minutes should be allowed before making any changes to the circuit
since there will be some zap left in the caps!

A single bleeder of 1/2 the resistance and twice the power rating could be
used. However, mounting R3 and R4 on the filter capacitors themselves
assures that they will still be safely discharged even if another part
fails or a connection opens up!

A capacitor charge indicator LED can be easily included for additional
protection. See the section: Visual
Capacitor Bleeder Circuit. An NC relay activated low resistance (say
1K, 20 W) bleeder could also be added to discharge the capacitors more
quickly whenever power is removed.

WARNING: For these line connected designs with a bridge rectifier, NO part of
the circuit can be tied to earth ground (as is possible with a HeNe supply)
for safety. Therefore, troubleshooting must be done with extreme care
especially if no isolation transformer is used. Connecting the ground lead of
a properly grounded scope to any part of the circuit will result in smoke or
worse! This applies not only to obvious component terminals but probably to
the HOT (thermally and electrically) regulator heat sink as well! See the
section: SAFETY When Dealing with Ar/Kr Ion
Laser Power Supplies.

To calculate the required values for R2, and C1 and C2, we can make the
following assumptions:

Maximum required Ar/Kr ion tube current is 10 A at 110 VDC. This should
handle any typical air-cooled tube you are likely to encounter. However,
figure 12 A maximum to allow some safety margin for the power supply.

Output of rectifier is 146 V peak minimum assuming a (low) 105 VAC input.
This comes from (105 x 1.414) - 2 V (for the bridge rectifier diode drops).
Thus, the power supply should continue to function at low line voltage (or
with your less than stellar house wiring)! Adjust appropriately if your
line voltage is fairly constant and known. High line voltage conditions
will result in more losses and will be harder on the regulator.

Ripple will be about 1 V per A per 10,000 uF of total capacitance. This
rule of thumb is based on a direct line-connected 60 Hz full wave/bridge
rectified resulting in ripple at 120 Hz. This is worst case and the 'pi'
configuration of the filter will reduce it still further - but we want to
be conservative. (Of course, if you have access to three-phase, this
capacitance can be reduced almost to zero - but that is another story!)

Note: Where an isolation transformer of marginal capacity is used for
testing, the peak capacitor recharge current will be limited and ripple
will increase. Therefore, don't be surprised if the discharge winks out
under these conditions. For this reason, an isolation transformer rated
for at least 2X of the laser power supply's maximum power consumption
should be used. Of course, such a transformer is HUGE!

Depending on the type of regulator, optimization considerations differ.
However, one common requirement is that the regulator still have enough
headroom even at maximum current (worst ripple factor).

For a linear regulator, we would like to minimize the drop across its
series pass-back to reduce the power dissipation in the transistors. The
total power dissipation will be the same but let the cheap, robust power
resistor (R2) do its fair share!

Based on this, one approach is to try to equalize the worst case ripple
and the drop across R2. This will result in reasonable values for C1 and
C2 while still pushing much of the power dissipation to R2. With 35 V
between peak and tube voltage (145 - 110) and subtracting out 5 V for
headroom, this leaves 30 V to play with. Dividing this in half results
in about 15 V of ripple, or from (3), above, about 4,000 uF each for
C1 and C2. R2 is then 1.25 ohm, 200 W (!!!). Trading off the values of
R2 and C1/C2 may be desirable depending on the specific needs.

Using an inductor instead of or in addition to R2 would require the use
of a part capable of passing the entire tube current - greater than 12 A.
However, it would run much cooler and reduce ripple more effectively.
Such an inductor can be constructed on the core of a salvaged microwave
oven transformer using several hundred turns of #14 wire. An inductance
of 6 mH provides an impedance of about 2.5 ohms at 60 Hz. However, it
is acting only on the AC component of the voltage on C1 so its effect
will be to reduce ripple to a greater extent but not reduce the DC level
as much as an equivalent pure resistance. This many turns of #14 wire
will also have a enough resistance that it cannot be totally ignored,
both in terms of its effect on the DC level and heat dissipation!

However, using just an inductor without some other means of regulation
may result in plasma instability or oscillation which may not be obvious
without looking at the current waveform on an oscilloscope. This will
damage the tube after a few hours of operation.

For a switchmode regulator, increasing R2 will reduce the voltage/power
requirements of the chopper transistors at full load. However, this will
negate some of the benefits of using a switchmode regulator since more
power will be dissipated in R2. The maximum on-time/duty cycle of the
PWM drive will need to be greater where the headroom is lower. And, the
dynamic response of the regulator will be affected and additional factors
may need to be considered to maintain control loop stability.

With R2 = 0, somewhat smaller capacitors can be used as long as adequate
current is available for the lowest points on the valleys of the input
waveform. Assuming 10 V of headroom is enough, this allows 25 V of
ripple resulting in C1 and C2 of 2,400 uF each.

WARNING: These HUGE filter capacitors can be lethal! 8,000 uF at 150 V is
about 90 W-s (90 J). Take extreme care when in operation and until you have
confirmed that the caps are fully discharged!

Where the Ar/Kr ion tube requires about 200 VDC, it makes sense to use s
either single-phase 230 VAC front-end similar to the type described above
or a three-phase front-end depending on power levels and availability.

Although the component values will change, the single-phase design is similar
to that described above for 115 VAC and the same design approach applies.

However, a simple buck autotransformer may be desirable to reduce the input
voltage somewhat to ease the dropping requirements of any series resistor
'heaters' and/or the current regulator pass-bank. A microwave oven high
voltage transformer can be modified for this purpose by removing the HV winding
and substituting a high current secondary putting out around 50 to 60 VRMS
(assuming a 230 VAC input and a 200 VDC tube drop) which is placed in series
anti-phase with the power supply input.

Note: 230 VAC is available from most residential wiring by using opposite
sides of the incoming power feeds. These come from a centertapped (utility
pole) transformer. Electric dryers, hot water heaters, stoves, central
airconditioning systems, and other high power appliances will already be wired
in this manner. Of course, safety warnings increase exponentially at these
higher voltages. There just isn't any room for error!

Going to three-phase makes sense for high power ion lasers requiring around
200 V across the tube if such power is available since for a given current and
ripple requirement, filter capacitor size (uF rating) is greatly reduced and
loading on the building's power distribution system is more balanced. This
approach makes sense where the Ar/Kr ion tube requires around 200 VDC at 20 A
or more - typical of lasers putting out many W of beam power.

The use of three-phase power greatly reduces the size of the necessary filter
components since the rectified pulses from the diodes are at a 60 degree phase
angle with respect to each other - 6 per cycle overlapping by 120 degrees.
So, raw ripple is down by around 85 percent even without any filter components
and the required capacitance on an uF/A basis is greatly reduced. In fact,
no capacitors at all may be acceptable assuming the power supply includes a
decent regulator. However, due to the generally larger current requirement,
an inductor makes more sense than a house heater size resistor in the 'pi'
network even though it needs to be a boat-anchor weight chunk of iron wound
with #10 AWG wire!

Note that regulator implementation may not change that much compared to those
used for single-phase 115 VAC systems with 100 VDC tubes. This is because the
voltage drop across a series pass linear or switchmode regulator can be set up
to be similar (at most requiring a modest size buck/boost transformer) in both
cases. Of course, current levels may be higher and protection devices need to
be sized accordingly for the higher total voltages and power levels involved.

If you really need these sorts of power levels, I am confident you will be
able to come up with the obvious extensions to these schemes as well as all of
the remaining nitty-gritty details. :-) However, here are some comments and
cautions on large-frame three-phase power:

(From: Dean Glassburn (Dean@niteliteproducts.com).)

If you do not have sufficient current and balanced voltage, specifically on a
large frame laser from (Coherent, Inc.) with
indium seals on the window stubs, the tube will see an undue amount of ripple
which in turn will heat up the anode to the point of melting the indium seals.
This is a common fault on large lasers of this type when one phase drops out.
Then you have a 500 pound boat anchor, no matter what you paid.

Most, if not all of the power supplies on three-phase units do not use the
Neutral as a load bearing wire. In fact, most Neutral wires are the same as
the grounding conductor (Safety/Earth Ground) at the service panel except the
latter is usually of a smaller gauge. The low voltages required for logic and
control in these supplies are usually derived off a common winding of the
three phases or off a separate transformer isolating the low voltage AC from
the mains.

Regulators - Linear, Switchmode, Control

For all but the most basic of 'testing' power supplies, some type of regulator
will be required to control current to the Ar/Kr ion tube. While there are
significant differences in implementation, many features are common to all
types of regulators:

Current rather than voltage regulation must be implemented. This is because
while the incremental resistance of the discharge is not quite negative
(which would be even more trouble), it is very low - a few ohms - so that
regulating the voltage would result in very poor control of current. A 5
percent change in voltage can easily result in a 50 percent change in
discharge current. Going from 5 to 7.5 A with a 5 V change would represent
an equivalent on-resistance of 2 ohms (but across a 100 V drop). Under some
conditions, current can still 'run away' if a fixed voltage input is
provided with no current regulator.

Several types of criteria may be used to regulate beam power:

Current feedback may be used to maintain a stable tube current which is
fixed in Standby mode or adjustable when operating in Current Control
mode.

Except for testing (where just a low value high power ballast resistor
and large Variac will suffice as long as current is monitored closely,
this current control is essential and sets the operating point providing
immunity from line voltage fluctuations and slight changes in tube
operating voltage (e.g., due to tube warmup).

Small series resistors or hall-effect devices are typically used for the
current sensors.

Light feedback derived from a photosensor monitoring the output beam is
used to maintain constant optical output (beam) power by controlling tube
current.

The DC coupled 'light control mode' is desirable to stabilize beam power
and maximize Ar/Kr ion tube life and adjusts the current around the
operating point.

An AC coupled 'noise signal' is also usually provided to reduce random high
frequency variations in beam power. These may be the result of plasma
oscillations or other instabilities possible with some laser head and/or
ion tubes.

An external modulation signal can also be used to vary beam power.

In all cases, the implementation usually uses some common op-amps and will
have loop compensation in the form of integral/proportional/differential
terms in the feedback network equations. Stability must be guaranteed for
under all conditions.

The regulator can be in the low-side or high-side of the power feed to the
tube. However, for a line connected system using a bridge rectifier,
NEITHER can be at earth ground so there are no particular safety benefits
to favor one over the other. Thus, all feedback and control circuitry must
be floating or coupled to the power circuitry via some sort of isolation
interface (opto-isolator or transformer). However, low-side regulation may
be preferred since it doesn't need to deal with the starting pulse (though
this is generally minimal).

A linear regulator controls the current going to the Ar/Kr ion tube in many
ways analogous to that of a common IC like an LM317. However, there are some
notable differences:

Due to the large currents involved, multiple large high power transistors
must be used. A typical pass-bank will require between 5 and 10 transistors
for a 12 A maximum tube. The transistors must be mounted in close thermal
contact to a large forced-air (or tap water) cooled heat sink since up to
several HUNDRED WATTS will be dissipated by the transistors at full power.
Since the collectors (cases) of the transistors are generally all tied
together, NO mica insulators are needed but a thin layer of silicone heat
sink compound SHOULD be used to minimize thermal resistance.

To reduce the thermal stress on the transistor pass-bank, its input voltage
can be reduced in a number of ways. There include the use of a high power
resistor (e.g, a heating element), a buck transformer (which could be a fat
winding added to the filament transformer), or a switchmode pre-regulator
(especially when driving an 80 to 110 V ion tube from a 230 VAC line).
However, in all cases, ripple on the main filter capacitor(s) and line
voltage fluctuations must be taken into account to assure enough headroom
under worst case conditions. With care, this would permit fewer and/or
lower cost transistors to be used. The latter two approaches would also
reduce total power dissipation.

WARNING: The entire heat sink will be line connected! DO NOT even think
about touching it (or anything else) until the unit is unplugged AND the
main filter capacitors have discharged!

Note: For testing and troubleshooting, mounting the transistors on the heat
sink with mica insulators (and silicone heat sink compound) may be desirable
to permit faulty or suspect devices to be easily isolated if necessary.
However, the mica will increase the thermal resistance a bit - the power
transistors will run hotter and/or more of them will be needed. On the plus
side, the heat sink can be electrically isolated as well - just don't touch
those TO-3 cans!

A plenum must be provided to guide the gale-forced-air from the power supply
cooling fan over the heat sink fins. This can be made of an insulator like
Plexiglas to also serve as protection from accidental contact with the
electrically and thermally hot heat sink and/or transistors.

Some commercial ion laser power supplies may specify very high performance
transistors. For example, the Lexel-88 uses 12 of the 2N6259 (apparently no
longer available) which is rated: 150 V, 15 A, 250 W! Motorola recommends
the 2N5631 as a replacement and their Master Selection Guide lists a couple
that are even a bit better: MJE15024 and MJE21194, but I have no idea of
price or availability.

NEC uses a pair of MJ11032s (ECG2349) - one as a preregulator chopper to
drop the voltage from the rectified AC line and the second as a linear
regulator for up to 9 A of tube current. The MJ11032 is rated at 120 V, 50
A (!!), 300 W (!!), and it's a Darlington so the minimum Hfe is 1,000 (!!).
This could be the ideal pass-transistor since so few may be required - if
you can afford it without taking out a second mortgage! I heard a price
quote of $17 in singles. :)

However, with care, it may be possible to use relatively inexpensive power
transistors like the MJE15015 (120 V, 15 A, 180 W) or possibly even the
ubiquitous 2N3055 (60 V, 15 A, 110 W) for the pass-bank. There are also
many types with specifications that are just slightly lower than the 2N6259
(mostly with respect to maximum power dissipation) so these may be worth
checking out (starting with a semiconductor cross-reference followed by a
search of your junk drawer).

Whatever route you choose, it is very important that the transistors in the
pass-bank be well matched in terms of Hfe. In addition, minimum Hfe at the
highest possible collector current be much more than adequate (say double)
to guarantee sufficient base drive. Otherwise, depending on the design of
the driver circuits, the collector currents could end up being seriously
unequal despite any emitter current balancing resistors. This may result in
significantly unequal stress (current and thus power dissipation) leading to
spectacular failures!

Protection for the pass-bank transistors must be provided in the event of
a short circuit fault or other transient event (possibly including the
initial spike when the tube starts). This may take the form of a circuit
to monitor the voltage drop across the transistors, preventing this from
exceeding safe limits. A prolonged overcurrent should shut down the
system. These are in addition to the required AC line fuses or circuit
breakers. A voltmeter may be permanently wired across the pass-bank as well
to provide continuous indication of transistor 'stress'.

The general circuit organization is shown below. In this example, the
regulator is in the cathode return between the centertap of the filament
transformer and the negative of the main rectifier/filter (DC-). Depending
on maximum required tube current, between 5 and 12 (or more) power transistors
may be needed.

A sophisticated transfer function for the feedback network, F(s), may be used
comprising separate loops for Current Control, Light Control (DC and AC - not
shown), Standby mode, and an external Modulation Input. See the section:
Multiple Loop Controller Organization for further details.

This regulator connects between the negative output of the main AC line
rectifier/filter (DC-, which is also the analog common) and the centertap of
the filament transformer (Tube-).

A minor variation on the basic approach using only discrete components is shown
in Simple Linear Regulator. This consists of a
two transistor differential amplifier, buffer, and pass-bank. This still
requires a floating +/-15 VDC power supply (not shown). Depending on the
size of the laser, anywhere from 5 to 16 pass transistors can be used. For
a small air-cooled type requiring about 100 to 110 V from a 115 VAC line,
2N3055s can be substituted for the high voltage expensive 2N6259s. While no
light feedback is shown, that capability can easily be added. Note that his
circuit has NOT been tested and a compensation capacitor may need to be added
at a strategic location to prevent it from oscillating (by reducing the
bandwidth of the pass-bank drive).

This regulator also has an input for a light control feedback signal but
additional circuitry would be needed to interface it to the typical laser head
light preamp output. (If a sensed light intensity signal is used directly,
it will only provide proportional control which is better than nothing but not
the integral/differential loop response required for best performance and to
suppress plasma oscillations. For that, the head preamp can be modified or an
op-amp circuit similar to the one shown on the SG-IL1 -
Control and Interlocks subsystem can be added in the signal path.)

It should be possible to simplify the regulator portion of the design even
further for use as an ion tube testing power supply by just implementing the
pass-bank and a massiveemitter follower. While this won't have the same
tiffness as a differential amplifier based control circuit, it should be quite
adequate for many purposes. This approach is shown in the schematic for the
SG-IY1 - Power Unit. This requires no floating
power supplies as the needed power is derived from DC+ and the voltage across
the pass-bank itself.

It is also possible to use a common 3 terminal regulator IC like an LM317 or
LT1084 in constant current mode. The advantage of this approach aside from
cost (these things are dirt cheap) is that the internal high gain circuitry
should result in extremely good regulation without many external components.
The IC Based Linear Regulator is adjustable from
about 2 to 10 A as drawn but may easily be modified for higher or lower current
capacity. A normal (35 V maximum) IC regulator can be used since the voltage
divider formed by R5 and R6 assures that the maximum voltage across the device
is never more than about 20 V (assuming a 50 V total between Tube- and DC-).
There resistors also provide the 'keep-alive' current (roughly 1.5 A) which
doesn't go through the pass-bank and is available immediately after the tube
starts even before the IC regulator kicks in. (This sets the minimum tube
current as well.)

If you have a bucket load of high voltage regulator ICs (e.g., LM317H), it
should be possible (if not entirely practical) to use several (like 10 or 20!)
in parallel on a BIG heat sink with current balancing resistors but no
additional pass-bank transistors. Details on this as well as adding light
feedback capability are left as an exercise for the student! :) (Hint:
replace R4 with some form of the light feedback signal.)

The line-connected (non-isolated) nature of most ion laser power supplies
introduces an additional complication where control of a linear regulator must
be accomplished via a PC, microcontroller, or other analog or logic circuitry.
This isn't an issue if the signal can be arranged to originate from a power
supply that is referenced to the regulator and there is no chance of human
contact with any portion of it including input signals. For example,
light feedback where the entire solar cell or photodiode and light sense
preamp can be floated may be powered in this manner. However, for something
like intensity control or DC coupled modulation, the simple solution is
probably not a safe option. In fact, with a bridge rectifier in the power
supply front-end, there is no way - safe or not - to interface the regulator
to a grounded controller without isolation.

One thought would be to use an opto-coupler. However, common opto-couplers
are not linear devices so using one will result in a non-linear transfer
function from the D/A or whatever and the output current. This probably
doesn't matter - a lookup table can take care of it if there is minimal drift,
but that isn't something that one can take for granted. One way to deal with
drift is to use two identical opto-couplers, one in a feedback loop to
linearize the overall transfer response.

Another way to provide isolation is to use a Pulse Width Modulated (PWM,
digital) signal via an opto-coupler or transformer. A simple low pass filter
and buffer amp will then produce an output which is proportional to pulse
width. Generating a PWM signal is a simple matter of comparing a linear ramp
with the input voltage - any of the PWM SMPS controller chips will do this
easily. For example, (though it would appear to be unnecessary), the Lexel-88
ion laser power supply couples the light feedback signal to its linear
pass-bank in this manner. See the chapter:
Complete Ar/Kr Ion Laser Power Supply
Schematics for the circuit details.

Rather than using the pass-bank essentially as a controlled variable resistor,
it is switched on and off at at high frequency - up to several hundred kHz.

A multiple L-C smoothing filter following this chopper removes ripple from the
resulting voltage to the tube. This is called a 'buck converter' because it
can only reduce the available voltage.

Since the switching device (theoretically) is always totally on or totally
off, it dissipates minimal power. In practice, the transitions take finite
time so this is not quite perfect. However, power loss in the transistors
IS much lower and overall efficiency is improved significantly compared to a
linear regulator. Higher switching frequencies permit smaller smoothing
components to be used. However, switching losses increase so there is (as
always) a tradeoff involved.

Due to the large currents involved, multiple large high power transistors
may be needed. However, since power is lost mostly during switching, fewer
devices can be used for a given power level. For a 'small' argon laser,
1 or 2 transistors will suffice and MOSFETs work well in this application.

You might ask: "Can large SCRs be used instead of transistors for the
chopper since they may be cheaper?" Good question. However, the answer is:
Not easily. The problem with SCRs is that you can't turn them off once they
are triggered whereas transistors go on and off at will. Without being able
to turn them off, you can't control the duty cycle and thus the power output.
There are some power supplies that do use SCRs in the front end but they
operate on the rectified AC - somewhat like a phase controlled light dimmer.
The switch-on point on the AC cycle can be controlled in that case.

Forced-air cooling is still probably required but at least the jet-engine
power fans are unnecessary (for the power supply - the laser head still
requires a hurricane to survive)!

Protection for the pass-bank transistors must be provided in the event of
a short circuit or excess current fault. A prolonged overcurrent should
shut down the system. These are in addition to the required AC line fuses
or circuit breakers.

The chopper uses one or more power MOSFETs. Since high current devices of
this type are readily available, a single part may be adequate for a 10 to 12
A regulator. However, they may easily be connected in parallel if needed.
These devices have internal reverse protection (Dx) and input clamp diodes but
additional protection is critical to prevent them from turning into short
circuits at inconvenient times (including switching spikes, power on/off
transients in addition to overload/fault conditions).

R1 limits peak current through the Qx bank into C2. Filtering is provided by
C2 through C4 and the associated L1 and L2. Since a high switching frequency
is used (e.g., 200 kHz), all of these components are quite small and compact
(at least relative to those required for 60 Hz filtering!). However, the
inductors need to pass the entire 10 A or more of tube current and the
capacitors need to be able to handle the high frequency high ripple current.
D1 is a 1000 V (typical) high current diode to isolate the igniter boost
voltage from this circuitry.

Most modern designs use an SMPS PWM IC like the SG3524 or UC3842. The major
internal functional blocks of these IC are the Sawtooth Oscillator (ramp
generator) and Voltage Comparator. Timing components (Rt, Ct) set the
(constant) oscillator frequency. The Voltage Comparator, subtracts the error
signal (Ve) from the instantaneous value of the sawtooth waveform. Its output
is high only if Verr is greater than Vosc. The pulse width is therefore a
linear function of error voltage over a fairly wide range.

The Buffer converts the output signals to drive the isolation transformer, T2.
Since the circuit is AC coupled, a stuck-at failure will result in the chopper
being disabled rather than full-on.

Note that while the simplified diagram, above, shows a single op-amp (and
single control loop for current feedback only), actual implementation may have
several. Since current is the actual controlled variable, this will be the
'inner' or 'primary' loop which is active as long as the tube is on in Standby
mode. The 'outer' or 'secondary' loops are responsible for user adjustable
Current Control, Light Control, and external modulation inputs. The Vcs
(Current Sense) signal is proportional to the Ar/Kr ion tube current. Vcl
(Current Level) is a voltage (negative in this case) from the front panel tube
current pot. See the section: Multiple Loop
Controller Organization for further details.

Typical oscillator frequency is 200 kHz. To analyze this circuit precisely
would require digital signal processing (DSP) techniques. However, where the
loop response is limited (by the Control Amplifier feedback) to much less than
the switching frequency, analog techniques can be used.

This approach is one way of providing an isolated means of monitoring Ar/Kr
ion tube current and is an alternative to the series sense resistor method
(which isn't isolated). The circuit consists of a sensor assembly which
includes a Hall-effect device located in the gap in a magnetic core with sense
and feedback coils.

The core has a 1 turn winding for the full tube current and a 1000 turn
winding (typical) for the feedback. The Hall Device (HD) is placed in a gap
in the core so that it intercepts the magnetic flux. The idea is to null out
the sum of the magnetic flux provided by the two windings to maintain tube
current at the selected level.

The voltage, Vcs (Current Sense), is proportional to the current required
to zero the magnetic flux and is thus proportional to the tube current which
equals (N * V)/R7 (where N is the number of turns in the feedback winding).
For example, with N = 1000 and Rcs = 100 ohms, the sensitivity is .1 V/A.

Without serious effort, some or all of the power transistors or MOSFETs in
both linear and switchmode ion laser power supplies will not survive an
output short circuit or even extended operation at excessive current. While,
it is possible in principle to design these in such a way that the pass-bank
components themselves are robust enough to ride out such faults, in practice
they would have to be extremely expensive - and it may not be something that
is desirable in any case as then the laser head itself might blow up rather
than power supply!

There are a variety of approaches one can take to handle these situations
without a lot of smoke and flames. However, they all consist basically of
three parts:

Control conditions to the pass-bank or chopper. Assure that ratings are
not exceeded during the early stages of a fault condition (before any action
can be taken).

The smoothing inductor in the chopper will limit the rate of rise of
current/voltage in a switchmode design. However, the transistors in a
linear pass-bank are often not able to hold off the full rectified line
voltage even for an instant. Thus, some means of limiting voltage across the
pass-bank long enough for shutdown to take place is essential. Of course,
this may mean that output current DOES climb above continuous safe limits for
a short time.

Sensing of a fault condition. Generally, either the voltage across the
pass-bank or the output current is monitored. Response time must be fast
enough that damage does not occur but be immune to the momentary transient of
tube starting.

Power supply shutdown. Once a fault is detected, the power supply must
shut down or enter an idle/fault state with the least trauma to itself and
the laser head. A series fast acting fuse or simply disabling the main relay
may not be fast enough. We all know that expensive devices blow to protect
25 cent fuses! A series power transistor or MOSFET switch is one
possibility.

Disabling drive to a switchmode chopper may be all that is needed if it is
fast enough. However, for linear pass-bank, input voltage will have to be
removed.

Pass-bank voltage or current (across a sense resistor) can be monitored with a
zener diode feeding an SCR circuit. When the SCR triggers, it can kill drive
to a series power transistor or MOSFET. Since this is hopefully a one-shot
(or at least only occasional) event, dissipation in the switching device isn't
an issue - only its ability to interrupt 10+ A. The essentially non-inductive
nature of the overall system makes this a somewhat less demanding task than it
might appear at first. There are a number of reasonably priced transistors
and MOSFETS rated at 20 A and 300 V that would be suitable.

For example, the Omni-150R, a switcher, includes an overcurrent shutdown
circuit which will shut off drive to its chopper MOSFETs, However, this may
not be soon enough to prevent their failure and that of several other
nearby components. The linear pass-bank in SG-IL1 is protected by limiting
its maximum voltage to about 60 V. A separate overcurrent shutdown circuit
should shut off main power - hopefully in time! But during a fault, lots
of unexpected things can happen and as they say: "This best laid plans of mice
and men....". :) In other words, the best advice even with fancy protection
is to make every effort to avoid major faults! See the chapter:
Complete Ar/Kr Ion Laser Power Supply
Schematics for circuit details.

The AC line front-end is similar to that described in the section: Single-Phase 115 VAC Line
Front-End. The filter capacitor, C1, should be selected to provide
acceptable ripple at full load but this is less stringent than for a linear
power supply or one without a regulator at all!

The chopper transistor, Q1, is a high power high voltage NPN power transistor.
Snubber/protection components are not shown. The drive signal must be passed
via an isolated interface since the emitter of the transistor is the line
connected DC-.

The rectifier on the secondary side, D5, must be a fast recovery type suitable
for the switching frequency used. The filter components can be relatively
small.

D6 must pass the full tube current and is used to allow the Boost voltage to
build up on the igniter circuit.

The PWM controller can use the same basic organization as that described in
the section: Switchmode Regulator
Controller. However, details will differ including the transfer
function(s) of the feedback network(s). These details are, as usual, left as
an exciting exercise for the student. :-)

While the vast majority of microwave ovens - perhaps every single one you will
ever see - use minor variations on the tried and trusted half wave doubler
circuit, a few models have been designed using solid state high frequency
inverters - in many ways similar to the deflection/HV flyback power supply of
a TV or monitor.

A typical circuit (from a Sharp microwave oven) uses full wave rectified
but mostly unfiltered pulsating DC as the power to a large ferrite inverter
transformer which sort of looks like a flyback on steroids. This means that
the microwave output is pulsing at both 60 Hz and the frequency of the
inverter!

The chopper transistor is marked: Mitsubishi, QM50HJ-H, 01AA2. It is a LARGE
NPN type on a LARGE heatsink. :-)

Note the similarity between the normal half wave doubler circuit and this
output configuration! Base drive to the chopper transistor is provided
by some relatively complex control circuitry using two additional sets of
windings on the inverter transformer (not shown) for feedback and other
functions in addition to current monitoring via the 'Sense' resistor in the
transformer return.

It is not known whether power levels in this over were set by the normal
long cycle pulse width modulation or by control over a much shorter time
scale. However, since the filament of the magnetron is powered from the same
transformer as the HV - just as in a 'normal' microwave oven, this may not be
very effective.

Compared to the simplicity of the common half wave doubler, it isn't at all
surprising why these never caught on (what is diagramed above includes perhaps
1/10th the actual number of components in a typical inverter module). Except
for obvious problems like a tired fuse, component level troubleshooting and
repair would be too time consuming. Furthermore, as with a switchmode power
supply (which is what these really are) there could be multiple faults which
would result in immediate failure or long term reliability problems if all
bad parts were not located. Schematics are not likely available either. And,
a replacement module would likely cost as much as a new oven!

This is simply a situation where a high tech solution was doomed from the
start. The high frequency inverter approach would not seem to provide any
important benefits in terms of functionality or efficiency yet created many
more possibly opportunities for failure. The one major advantage - reduced
weight - is irrelevant in a microwave oven. Perhaps, this was yet another
situation where the Marketing department needed something new and improved!

Multiple Loop Controller, Light Feedback, Overcurrent Trip

The general circuit shown below can be used (with just a bit of refinement!)
for any of the regulators (linear, switchmode, inverter). A number of signals
affect the output of the controller which is sent to the regulator itself:

Vcl - V(Current Level) - A level proportional to the desired tube current.
This is from a front panel control.

Vcs - V(Current Sense) - A signal proportional to the anode current.
This is typically derived from a series resistor or Hall-effect sensor.

Vll - V(Light Level) - A level proportional to the desired optical output
power. This is from a front panel control.

Vls - V(Light Sense) - A signal based on optical sensing of beam
intensity using a photodiode or solar (photovoltaic) cell. DC and AC
components provide power level control and optical noise reduction,
respectively.

Vmi - V(Modulation Input) - An external signal that can be used to vary the
tube current about its set value when operating in Current Control mode.

Vsb - V(StandBy) - A signal which goes high during standby to cut tube
current. This may also do other things like reduces the loop gain (not
shown).

In the diagram, below, all inputs are positive going signals or levels.

The Primary (inner) loop feedback network (IC1C) consists of Rdc to provides a
DC set-point (proportional) for the tube current based on the Vcs (current
Sense) feedback signal and P(s) which is in the form of one or more series R-C
networks in parallel with Rdc to control loop frequency response. This
results in a proportional-integral loop response.

The Secondary (outer) loop feedback networks, C(s) (Current, IC1B) and L(s)
(Light, IC2A), typically consist of one or more series R-C networks in
parallel to produce an integral response characteristic. Stability must be
assured for any combination of Current and Light front panel control settings.
(With some designs, it is possible to destabilize the loop by turning up the
Noise Gain pot (Rng) too high.) In the circuit, above, D3 and D4 are included
to clip the AC component of the light signal to prevent this from happening.

Possible modes are as follows:

Current Control only - S2 closed (disables Light Control). Tube current is
determined by current sensing (Vcs) alone based on the front panel Current
Level pot (Vcl). Maximum current is determined by the Current Limit pot
(Rcl). (This is in effect for all modes except Standby which has its own
lower setting.)

Light Control - S1 closed (disables Current Control). Tube current is set
based on light sensing (Vls) to provide the desired optical output based on
the front panel Light Level pot (Vll).

The Noise Gain pot (Rng) adjusts the extent to which this AC signal is used
in reducing optical noise while D3 and D4 clip the noise signal to prevent
loop instability.

Note: S1 and S2 are actually optional as the same effect can be achieved by
turning the associated front panel controls to their lowest (counterclockwise)
settings effectively disabling Current Control or Light Control respectively.
is too high and vice-versa. Normally, one or the other of these pots would
be left in the fully counterclockwise position to disable that control loop.
Whichever loop has a higher error voltage (at point Verr) will be in control.

Modulation - Vmi may be enabled by closing S3 when operating in Current
Control or Light Control mode.

Standby - The purpose of this mode is to operate the tube at a keep-alive
current which avoids any startup delay but provides maximum tube life. S1,
S2, and S4 all closed (and S3 open) enable Standby mode. The Standby Adjust
pot sets the current in this closed loop implementation.

The Solar Cell, SC1, generates a current which is proportional to incident
light from a portion of the laser beam. The Op-Amp, IC1, converts this to
a voltage which is buffered by Q1. C1 limits the frequency response to
assure loop stability. The Op-Amp's output is applied to the control
amplifier to maintain beam power stable based on its actual intensity.

A PIN photodiode could be used instead, appropriately biased to inject current
into the amplifier input.

Both the tube and power supply must be protected against excessive current.
In some cases, this will be on a time delay. The overcurrent trip circuit
below will respond in a time determined by the values of the components in the
input filter network and severity of the overload.

Igniters, Pulse, Resonant, Bypass, High Current High Voltage Diodes

This is a high voltage pulse generating circuit with high current pass-through
capability and has some features in common with the pulse starting circuits
for HeNe lasers.

It is similar to the igniter used in the Omnichrome 150R power supply
and 532 laser head (see the chapter: Complete
Ar/Kr Ion Laser Power Supply Schematics for details) and is intended to be
placed in the high-side (anode circuit). However, some designs may put the
igniter in the low-side (cathode circuit) instead.

The first part of the circuit is used to generate an approximately 400 VDC
'boost' source from the AC line. This is a classic voltage multiplier. The
Boost output is used to charge the energy storage capacitor (C6) for the pulse
circuit and power the relaxation oscillator that triggers it repeatedly until
the Ar/Kr ion tube starts. The supply voltage to the relaxation oscillator
(across D6) is then automatically removed and triggering ceases.

The primary reason to use the boost voltage rather than the 150 VDC available
from the line rectifier/filter at DC+ is to dump some additional energy into
the tube at the instant of startup (from C8) to aid in transition from a glow
discharge to the high current arc required during normal operation.

Another reason for using the higher boost voltage is to reduce the number of
fat wire turns on the pulse transformer toroid. Increasing the voltage on the
energy storage capacitor from 150 to 400 V reduces the turns ratio by better
than 2.5:1 requiring only 30 instead of 80 turns on the secondary. Once you
have wound one such transformer, you will appreciate this savings!

I've used the ferrite core of a deceased flyback transformer for T1 with a 2
turn primary and 30 turn secondary. This worked fine for my home-built
Cyonics tube based laser head. The large ferrite cores from PC (or other)
switchmode power supplies should be fine as well. Make sure the secondary
can handle the full ion tube current and is adequately insulated for the
several kV or more that is produced.

The bypass capacitors, C9 and C10, complete the return circuit from the bottom
of the secondary of the igniter transformer to F1 (of the ion tube
filament/cathode). Without these, in addition to interfering with starting,
this pulse could find its way back into the power supply itself resulting in
cascade failures of regulator or other components.

One concern with a pulse igniter is damage that may occur to the tube cathode
each time the igniter fires. This is generally not a serious issue if
properly designed with minimal energy in the pulse but over the long term,
may have some effect on the life of the tube. And, it could be particularly
hard on tubes that take many tries to start. It is possible to
design an igniter that is more gentle. Some ion lasers use a resonant
igniter which is in essence a circuit like mini-Tesla coil that generates
a burst of radio frequency (RF) rather than simply a voltage pulse. The
amplitude of the RF pulse can be much more modest than of the HV pulse
and since using a Tesla (or Oudin) coil is one way to start an uncooperative
tube, such as approach does make sense.

While the typical circuit does generate a high voltage pulse, it doesn't
actually appear on the tube anode but is used to jump the spark gap of a
resonant (LC) circuit, sort of like an early spark trasmitter. :) The
spark gap discharge completes the series LC circuit which then produces
a damped RF oscillation and that ionizes the gas in the tube, allowing
the main DC current to flow.

See the schematic of the
NEC GLG3030 - Laser Head. As can be seen, the
igniter transformer, T1, has a pulse generating autotransformer winding on
the left, and the resonant winding on the right. The SCR, (SCR301) discharges
C301 into the primary which generates a high voltage pulse in its
autotransformer secondary. This causes the spark gap (UDB-3) to break
down allowing C303 to resonate with the series winding of T1, through
which the main tube current also passes.

While superficially, this looks similar to the typical pulse igniter, the
resonating winding of T1 is designed along with C303 to produce the burst
RF at a relatively low voltage, which is easier on the tube cathode.

Note that even the pulse starter in the previous sections has a small
capacitor on the igniter transformer secondary, so it too may have some
RF component in the output, just before the boost capacitors discharge
through the tube and whack the cathode! :)

Now here's a strange igniter. This unit was found in a Hughes JVC model
100 video projector and is used to start the arc lamp (rather than an
ion laser) but that's just a detail. :) Its high voltage
source is a standard Melles Griot helium-neon laser power supply brick, an
05-LPM-949, typically used for a 5 mW (rated) HeNe laser. This feeds a
potted unit which contains a 60 AMP transformer, with an external high
voltage capacitor (0.01 uF, 10 kV) and spark gap (5.5 kV). The input
to the HeNe laser power supply is the raw DC for the arc lamp, even
though this brick normally runs on 115/230 VAC.

Apparently the way it works is as follows: Before the arc lamp strikes, the
voltage across it is about 150 VDC. This would seem to be lower than the
normal internal DC voltage of the HeNe laser power supply (after the 115
VAC or 230 VAC line voltage is doubled or rectified and filtered) but
regardless, is enough for it to try to start the non-existent tube by
generating the 10 kV start voltage, charging the HV capacitor until the
spark gap breaks down at 5.5 kV. That results in some combination of a
high voltage pulse and high frequency oscillation just as in the resonant
igniters described in the previous section, with up to 32 kV
available to start the lamp. Once the arc lamp
is lit, its sustaining voltage is only around 19 V (at 39 AMPs!),
insufficient for the HeNe laser power supply to do anything, so it twiddles
its thumbs during the movie. :-) Interestingly, the 05-LPM-949 has *two*
adjustment pots, with the second one being to set the startup delay. The
extra trimpot may be the only difference between the 05-LPM-949 and the
otherwise similar 05-LPM-948.

I can't imagine this to be a less expensive solution than one using a
miniature high voltage power supply module designed to be a high voltage
power supply module, rather than a HeNe laser power supply.
So, it's either an elegant ion laser igniter on steroids - or a kludge -
depending on your point of view. ;-)

An ALC-60X/Omni-532 or other large tube (e.g., a Lexel-88) needs considerable
energy to form the cathode spot. And, over time as the pressure goes down it
WILL need the high power resonant ignition approach described in the section:
Ar/Kr ion tube pulse type igniter.

However, a small, modern, tube like the Cyonics starts rather easily with its
short bore and oversized cathode. Therefore, it may be possible to use a
simpler approach for its igniter using a low current high voltage (say 2 kV)
supply feeding onto the anode side of a HV bypass diode as shown below:

R1 limits current from the HV supply while R2 limits current from C1 at the
instant the tube starts. D1 allows the HV to build up across the tube. Of
course, high current diodes with 3 kV ratings aren't cheap either! But, see
the section: Construction of HV High Current
Blocking Diodes.

I have not tested this circuit but promise to do so in the future. While this
general approach works well for starting HeNe tubes, they are not quite the
same animal!

The ALC-60X/Omni-532 design does use a diode switch like this for the boost
voltage but I have only seen xenon arcs using it for starting HV. I suspect
there is a impedance problem - i.e., you get a glow that doesn't progress to a
arc and the cathode spot doesn't form. Arc lamp supplies that use this method
often overcharge the caps in the power supply to insure they start as well as
use a trickle supply of about 1 kV at several mA while running to keep things
ignited.

However, for an older or larger tube, you really have to hit it with the pulse
igniter.

Filament and Low Voltage Supplies, Line Frequency Clock

This is implemented as a simple low voltage high current power transformer
with a centertapped secondary winding. Typical voltage is 2.5 to 3.0 VRMS
at 15 to 25 AMPS. Make sure your connections are secure!

To balance the current feed through the Ar/Kr ion tube cathode, the negative
from the power supply is applied to the center tap of this winding. Since the
low voltage secondary has only a few turns of fat wire and is an outer winding
if not on its own bobbin, it is a simple matter to add a centertap if you are
modifying an existing transformer and one doesn't already exist.

Since this voltage must be relatively accurate, a Variac or some other means
of adjusting it needs to be provided. Commercial designs typically provide
multiple taps on the transformer primary to set up the proper current. With
a semi-homemade secondary, partially unwinding one turn may be all you need to
tweak the current.

Salvaged microwave oven transformers with the HV windings removed work well
in this application. Remove the high voltage secondary winding (hack it off
if necessary) so that you don't have this additional danger! The filament
winding may need to be adjusted in size (number of turns) and a centertap
will need to be added. See the section:
Rewinding a Microwave Oven Transformer for use
as a Low Voltage Filament Supply.

A standard high current power transformer can also be used. For example,
the Stancor P6433 (actually called a 'filament transformer'!) is rated at
5 VRMS CT, 15 A. While these specifications are not exactly ideal, it
should work well when powered from a Variac. (However, this specific
dinosaur may no longer be available.)

Note that while some commercial ion laser power supplies claim to use
DC for the filament to reduce ripple and noise in the laser output
(Melles Griot 176B, for example), this is not recommended even if you
have a suitable low voltage high current DC power supply available:

(From Steve Roberts (osteven@akrobiz.com).)

DC requires some changes to the cathode-to-bore spacing (longer), or a
slowly wandering DC offset from inside the switching PSU, or you end
up with a hot spot. The only DC cathode tubes I know of go into very
precise semiconductor wafer measurement stuff. I got a call from one
of the major tube makers about two years ago asking for advice, it
seems their main plasma guy had left and they wanted to find another
way after X heads on DC test killed their cathodes, periodic polarity
flipping becomes a option too. For long Life, AC rules.

These can be of conventional design using a low voltage centertapped power
transformer, bridge rectifier, filter capacitors, and a pair of IC regulators.
For typical +/-15 VDC analog power, suitable regulators can be the common
7815/7915 types. Logic power (+5 VDC) can be obtained from the input to the
V+ using a 7805 IC regulator and dropping resistor to reduce its power
dissipation.

The only major consideration is that one or more supplies of this type may be
needed that are electrically floating to power the regulator controller and/or
light feedback circuitry if they are direct coupled to the regulator (which is
on the line connected DC+ or DC- feed). However, this is easily solved since
any decent power transformer will be rated for at least 2,500 V isolation.

Single point connection between analog and digital commons is made at point X.

In addition, place .1 uF ceramic capacitors across each of the electrolytics
to bypass high frequency noise.

WARNING: If these are floating - not at earth ground - there will be blown
parts and vaporized wiring if connected there!

Depending on the current requirements, the regulator ICs will likely need to
be mounted on heat sinks (isolated from each other using mica spacers and
silicone heat sink compound if on the same one).

The circuit as shown above is rated at about .5 A for each of the 15 V outputs
and an additional .5 A for the +5 V output (using a nice heat sink for the
IC regulators!). R1 should be selected to leave about 2 or 3 V of headroom
at maximum logic current to reduce power dissipation in the 7805 chip.

An isolated 60 Hz clock can be easily extracted from the secondary winding of
any of these low voltage power transformers:

The opto-isolator, OC1, can be either a photodiode or phototransistor (shown)
type although the value of R1 may need to be adjusted based on this and the
transformer's output voltage. Using a bridge or full wave rectifier in series
with OC1 instead of D1 across it will result in a 120 Hz clock. However, the
lower frequency clock is probably better for the most likely use - a timer for
the filament preheat delay.

Requirements, Safety, Protection, Switches, Indicators

The schematic in U.S. Patent #4,504,951: High Speed SMPS for a Light
Controlled Laser System (ALC), is a reasonably simple design. Actual
production units have about twice the parts so as to get a reprographic or
instrument grade beam. See the chapter:
Complete Ar/Kr Ion Laser Power Supply
Schematics.

For a light show, an ion laser power supply just has to do the following
(assuming an ALC-60X/Omni-532 class tube - others will use different values):

Warm up the cathode and charge the caps slowly before starting. Most
supplies use a CD4040 chip to count 5,000 or so pulses of the 60 Hz line
for a 35 second delay while others use a 555 timer as a monostable.

Provide 4.5 to 10 A constant current (adjustable) into a tube that drops
100 to 110 VDC but acts (incrementally) just about like a dead short.
Adjusting the current from 4.5 A to 10 amps varies the beam power from 20
to 110 mW (for the ALC-60X/Omni-532).

Power up the fan(s).

Provide current regulation to compensate for line voltage load impedance
variations (as the tube heats up).

Monitor the over-temperature and case safety interlocks, and check for the
presense of the fan(s) via a wire loop on the fan connector. Refuse to
power up/shut down should any of these be open.

Shut down if tube current exceeds 9.50 amps for more then a minute.

Provide light control circuitry for beam stabilization and diagnostics.

While regulation (4) and overcurrent sensing (6) are not strictly required,
bad things can happen if tube current is not monitored continuously. Light
control (7) is also optional but Ar/Kr ion tube life will be shorted slightly
without it. However, for external mirror lasers, the light sensor provides a
great way to peak the mirrors as the light sensor signal shows up on banana
jacks on the side of the head, along with a pair of jacks for the tube
current, so diagnostics are built in to the head at the factory (or should be
by you if you are rolling your own).

The following are essential for both personal safety as well as protection of
the equipment in the case of an electrical or cooling failure:

Equipment MUST be supplied with a #14 AWG or larger gauge 3 wire grounded
cordset and plugged into a properly grounded 20 A (minimum) circuit. While
a dedicated circuit is not required, don't figure on powering anything else
at the same time you are using your laser. When cranked up to maximum
power, AC line current will be close to 15 A. Of course, if you aren't
going to run at full power....., OK, that was a really silly statement. :-)

Fuses or circuit breakers for the following power feeds should be provided:

Main power (line input): 20 A.

Preheat power: 3 to 5 A.

Power supply fan: 1 to 2 A.

Head fan: 3 to 5 A.

Logic/analog power: 1 to 2 A.

Light control (optional) power to head: 1 A.

Thermal protectors should kill main power (but not fan power) where
temperature limits are exceeded on the.

Ar/Kr ion tube assembly.

Power supply regulator heat sink.

The fan power should be maintained until the relevant assembly's temperature
drops to guaranteed safe value.

Interlocks should prevent operation or kill main power if:

The laser head case is opened.

The power supply case is opened.

The laser head fan is unplugged.

Cheaters can be provided for testing - but only for testing!

Logic using relays, TTL, CMOS, or vacuum tubes :-) must be provided to:

This is just a series connection of all the required interlock switches,
plugs, and thermal protectors - some portions of which have indicators on
them. A single lamp would suffice across the entire string (just an NE2H in
series with a 47K resistor for a 115 VAC interlock circuit!) but this tells
you exactly where the problem is located. However, a special sensitive
indicator would be needed to work properly where multiple interlock are open
simultaneously (and more than one indicator would end up in series - dimming
or not lighting up at all). See the section:
Interlock and Protector Indicator.

If ANY switch or interlock opens during operation or is open before powering
up, power to the Ar/Kr ion tube should be shut off or be prevented from
coming on. Control power should remain active and the relevant interlock
status indicators (if present) should light up.

In order to assure personal safety and protection for the components in the
power supply and laser head (particularly, the Ar/Kr ion tube), interlocks
and sequencing are required. This can be done partially manually (for
testing) but automatic (digital) control will be needed for any sort of
permanent setup. There are a couple of approaches one can take:

Basic logic using a combination of TTL or CMOS devices, timers (e.g., 555 or
CD4040 counter), and mechanical or solid state relays. This is what we will
describe.

Microcontroller or PIC based system running a program in EPROM. If you
want to use this 'high-tech' approach, I am confident you will figure it
out! :-)

Note that in both cases, certain key functions should be hard wired such as
safety interlocks and thermal protection for the tube. Do NOT depend on
electronic control for these! Furthermore, there must always be the
equivalent of a FAT RED stop or off button - and it must have a direct effect
even if some stupid TTL device or firmware program is locked up!

While everything below isn't essential for a bare-bones hobbyist supply, these
features will add a professional touch to your system! The functions that
need to be provided are as follows (these are all outputs or states):

Note: some tubes may be happier if their cooling fan is shut off at the same
time as power or shortly thereafter rather than waiting for full cool-down.
Thus the 'Head Warm' switch (thermostat) may not be needed (or can provide
reduced airflow after the tube has been shut off).

(From: Steve Roberts (osteven@akrobiz.com).)

The 60X generally will withstand a sudden shutdown, in fact rapid cool-down
with the fan on is more of a lifetime shortener. This is not true of all
air-cooled lasers, some of which store too much heat and need the fan. I
generally just shut everything down at once with no problems in years of
operation. More modern supplies run the fan about 1 minute after shutdown.

Component Selection and Construction Considerations

I would recommend a very conservative approach to parts specification.
Clearly, this is critical in the power systems where a failed component can
result in smoke, fire, and explosions which can easily ruin your whole day.
However, a fault in the logic or analog control circuitry can result in
equally disastrous consequences since the regulator is driven from there.

For the power handling components, derate resistors, capacitors, and
semiconductors by 30 to 50 percent. For example, where the main bridge needs
to be rated for 10 A, use a 20 to 25 A device - the increase in cost will be
minimal and well worth it. Electrolytic capacitors should be 200 V minimum,
250 desirable for the line filter. Be particularly conservative with the
regulator power transistor ratings. Figure on at least 5, 150 W power
transistors in parallel for a linear pass-bank handling 10 A even though the
individual devices may be rated for 15 A.

For the logic and analog circuits, use high quality name brand components (no
unmarked op-amps you happened to have in the junk bin!). Make sure you follow
the recommended practices of providing bypass capacitors where needed (and on
all logic devices), tie unused inputs to a legal state, etc. Separate digital
and analog grounds except at a single point. Route digital and analog wiring
separately.

Here are some guidelines (well, some are more than guidelines for your own
safety at least) for the power supply, laser head, and general construction
practices.

For the power supply itself:

Build the entire power supply into an enclosure such as a large metal box
intended for electronic equipment or rack. Portions (like the top or sides)
can be made from clear plastic if you want to show off your creation.
All user accessible metal parts must be connected to the grounding conductor
of a #14 AWG or larger (lower AWG number) 3 wire heavy duty cordset and it
must be used ONLY with a properly grounded 20 A outlet.

Securely mount all heavy components like transformers/chokes, large power
resistors, and computer grade electrolytic capacitors, with properly sized
nuts, bolts, and lockwashers or clamps as appropriate.

Mount the regulator power transistors on a proper finned heat sink with the
fins oriented parallel to the air flow for optimum cooling.

Provide adequate flow-through forced air cooling for power semiconductors
(especially the regulator transistors) and high power resistors. A plenum
may need to be fabricated to guide the air flow over the regulator heat sink
and other critical components.

Specifically for the laser head:

The laser head can be constructed in a case made of entirely of metal or
with portions made of plastic (for visibility, etc.). There MUST be a solid
safety ground connection (#14 AWG or larger) between any user accessible
metal parts of the case and the main power supply safety ground.

Make sure all components - especially the Ar/Kr ion tube (and any associated
optics), filament transformer, and other heavy or precision components are
securely mounted with access for replacement and/or adjustment where needed.

Keep wiring between the igniter, filament supply (transformer), and tube as
short as possible. Provide 1 inch or greater separation between tube anode
wiring and anything else and/or use wire with 25 kV insulation or better.

A HUGE cooling fan (typical minimum requirements: 225 cfm) must be provided
to draw air through the cooling fins of the tube. A plenum may need to be
fabricated to couple the fan to the tube. Make sure that NOTHING obstructs
the cooling air. Route wiring to keep insulation away from the hot tube and
its fin structure. Keep in mind that you must be removing about the BTU
equivalent of a modest size space heater from your laser head!

Don't forget the laser and electrical safety warning labels!

General:

Small parts can be wired point-to-point between larger ones (e.g., the screw
terminals of computer grade capacitors, bridge rectifiers, chassis mounted
power resistors, power transistors) or heavy duty terminal strips.

For connections to high power dissipating components like large power
resistors, use nuts and bolts WITH lockwashers or lugs rated for high
temperature use and crimped using the proper tool. Use wire with insulation
rated for high temperature (heating appliance) use on connections to these
components as there is considerable conducted heat transfer.

Construct all the low power circuitry (digital and analog) on perforated
style prototyping boards or printed circuit boards (PCBs). Use only new
high quality double-wipe or machine pin sockets for ICs - solder everything
else directly to the board.

CAUTION: Keep in mind that portions of this circuitry will very likely need
to be floated electrically since it will be line connected (via DC+ or DC-).
Therefore, make sure any copper cladding is peeled away from screw locations
and/or non-conductive standoffs are used for mounting.

Provide fool-proof interlocks (it is amazing how many fools are out there!)
to kill power in the event that ANY covers or access plates are removed or
opened. You can always construct cheaters for testing but only for testing.

Special Parts 1 - Low Ohm Resistors, HV High Current Diodes

Many of the resistors listed in the Ar/Kr ion laser power supply circuit
diagrams and complete schematics call for ratings of a small number of ohms
or a fraction of an ohm - but at 10s of watts, 100s of watts - or greater!
While it may be possible with great effort to locate these from electronic
distributors around the globe or beyond (and searching that far may be
needed!), they can often be constructed for next to nothing in total parts
cost. And, there may be much more flexibility when what you really want is
not a standard value!

Incandescent light bulbs are the usual low cost solution for troubleshooting
of electronic equipment to provide current limiting. However, the resistance
of their filament can vary by a factor of 10 from cold to hot - not good for
our purposes. However, there is a nearly as readily available alternative:

The elements from space heaters, electric dryers, and other heating appliances
with exposed coils are ideal for the fabrication of home-brew but perfectly
usable power resistors. These are made of NiChrome, an alloy of nickel and
chromium which is resistant to oxidation even when yellow/orange-hot and its
resistance is relatively stable with respect to temperature (which cannot be
said for tungsten light bulbs).

Space heater elements are usually rated at between 600 and 1,500 W at 110
VAC for an equivalent resistance of 20.2 and 8.1 ohms respectively for the
entire element. Oil filled 'radiator type' heaters are nice because they
usually provide multiple power (resistance) settings AND run cooler which
means their resistance characteristics should be more stable.

Electric dryer elements may be 2 to 3 times the wattage running on 230 VAC
(resistance may be lower or higher depending on type).

Even burnt out elements - readily available from your junk pile (or that of
the local appliance repair shop) will be 99 percent undamaged. They are
usually broken or melted through in one spot and what remains is ideal for
creating custom and/or adjustable high power ballast, dropping, or sense
resistors. Deterioration of the overall wire is generally minimal.

The original mounting can be used or portions of the element(s) can be
transferred to a suitable non-conductive and NON-FLAMMABLE support. Allowing
the wire to come in contact with this material at as few locations as possible
will minimize heat transfer and make the most effective use of air cooling.

Taps can be provided for easy adjustment. Initially, these can make use of
heavy duty crocodile clips later made permanent with nuts and bolts or crimp
connections (soldering won't be reliable, surprise, surprise!).

For current capability approaching the original application, paralleling
multiple heating element wires will permit them to run cooler.

Sealed heating elements of the types used in electric stove tops, ovens,
broilers, electric hot water heaters, etc., may also be used but there is no
way to adjust their value except by switching in various series and parallel
combinations. However, this is convenient with dual-element stove top
'burners' with pushbutton selector switches (not thermostats or 'infinite'
controls - your 1950s vintage GE range probably has what is needed!).

CAUTION: Any element designed for direct immersion in water or another liquid
may burn out even at far less than its rated current if run in air despite a
cyclone of cooling!)

I don't know if you can find something like a 3 kV, 12 A diode at major
electronic distributors. However, they can be constructed from multiple
series strings of smaller diodes in parallel along with resistors to balance
the current.

For example, for a 10 A power supply, use 2 series strings of four 1000 V, 6 A
diodes with a .05 ohm, 5 W resistor in each string as shown below:

Special Parts 2 - Relay Drivers, Indicators

Relays with AC coils are convenient for direct control from the power line
or a power transformer secondary. The most common coil ratings are 24 and
115 VAC, 50/60 Hz.

Relays with DC coils are convenient for control from low voltage logic
circuits. The most common coil ratings are 5, 6, 12, 24, and 120 VDC.

Solid state relays are a combination of an opto-isolator and triac or other
semiconductor switching device - no moving parts! These are driven directly
from a low current (TTL) logic signal but are usually more expensive than
their mechanical counterparts. Also, being solid state, they may be less
robust under certain (stressful) conditions!

There are several options depending on how these are to be controlled:

When front panel switches or other relay contacts are used to activate a
relay, the simplest approach is to use the 115 VAC (or 24 VAC if you have
that from the low voltage power supply transformer) directly.

For logic control, a small DC relay can be used to provide switching for the
AC coil of a larger relay. This is the simplest and easiest approach. See
the section:
Driving Relays with DC Coils for circuit
options.

Alternatively, a circuit equivalent to a small solid state relay can be
built using an opto-isolated triac in series with the relay coil and the AC
line. By using a device with zero-crossing switching, no additional parts
are required:

CAUTION: Snubber components (a series RC network, not shown) may be required
across the triac to limit voltage spike amplitude when the device switches
off if a non-zero-crossing type device like the MOC3012 is used.

Small relays are designed to be connected directly to a TTL output. These
will have 5 VDC coils rated at a current as low as 10 mA or less. Larger
relays with 5 VDC coils may require as much as 100 mA or more so a beefier
driver (beyond a TTL logic gate) may be needed for some of these. Relays with
higher voltage DC coils can also be used. For a given type, coil current is
inversely proportional to coil voltage. Therefore, using a relay with a
higher voltage coil may be easier if a suitable power source is available.

As always, when driving an DC inductive load, a back biased 'free wheeling'
diode is placed across the coil to provide a path for the coil current to
continue flowing (and ramp down to zero relatively slowly) when the driver
switches off. This prevents any inductive voltage spike which would result
in stress on the driver and/or logic circuits and may generate excessive
electrical noise.

The following are several alternatives for driving these relays (12 VDC, 10 mA
coil assumed; typical contact configuration shown):

For the 4N33, the current transfer ratio is 500 percent so to drive a relay
with a 20 mA coil requires a minimum of 4 mA through the LED. Add another
buffer transistor to drive a higher current relay coil.

There are several places in an Ar/Kr ion laser power supply where status or
warning lights are required and there are no convenient sources of low voltage
for LEDs.

In the good old days, everyone used neon indicators. The problem with neon
indicators aside from the fact that they invariable start to flicker after
a few years of constant operation (which really shouldn't be a problem here)
is that you have any choice of color as long as it is orange. :-) However,
these really are the cheapest and easiest solution for putting an indicator
on a power line:

To use an LED on a 115 VAC circuit, it is better to use a capacitor to limit
the current than a resistor as power dissipation is greatly reduced. The
following circuit will work with minor modifications in component values for
most LEDs (4 or 5 mA assumed as drawn):

D1 bypasses reverse current and R1 is for surge limiting to prevent any
possible harm to the poor little low voltage LED if you apply power when the
AC input is near its peak.

Using a small bridge rectifier (almost any voltage rating) would double the
brightness for the same value of C1 (assuming the LED can handle the current)
but this hardly seems worth the effort for a simple indicator!

Note: For both these circuits, a high value resistor - say 1 to 10 M ohm - is
recommended across C1 or the input (not shown). This will quickly discharge C1
when power is removed. The energy C1 can hold isn't going to kill you, but
all those little shocks can add up to a lot of @#$% words!

Where a single indicator is adequate on circuit containing one or more series
connected normally closed interlocks or protection devices, a simple lamp
across the entire chain will suffice. However, were separate indication is
desired, this will either not work at all or result in varying brightness
levels depending on how many of the links in the chain are open. So, after
your power supply is basically operational, consider spicing it up a bit with
individual lamps for each possible fault condition - just like NASA!

Even with several (e.g., up to 4) of these devices in series), enough current
should flow with any combination of them open to light their respective LEDs
at nearly the same brightness. However, maximum bypass current will well
below the minimum needed to activate the controlled relay.

+-+
On the interlock wiring diagrams, this type of indicator is shown as: --|I|--
+-+

The circuit below can be used to provide a solid visible indicator for each
normally closed (NC) interlock or protector in a series 115 VAC relay coil
circuit.

As drawn, maximum AC current to this circuit is about 3.5 mA. Check your
relay coil specifications to determine if this will result in possibly
activating or not releasing the relay, If so, you will have to reduce
the value of C1 and adjust other components accordingly (an opto-isolator
suing a photo-darlington may be needed). Or, provide a load resistor from
AC Hot to the top of the interlock chain to increase the available current
(bypassing the relay coil).

For use on lower voltage AC, adjust the values of C1 and R1.

For use on a low voltage DC circuit, remove C1 and D1 and select R1 to
provide appropriate current limiting to OC1.

For a value of Vcc other than +5 V or for a different LED, change R2 to
select the proper LED current.

A non-isolated alternative for a low voltage DC circuit (e.g., +15) is shown
below. This implements a constant current LED driver. It requires as
little as 3 V across each driver and supplies a constant 5 mA to the LED.
Modify for your own needs.

Note: Check the specs for the coil of the relay you intend to use. If its
holding or activation current is close to 5 mA or less, this circuit will
need to be modified. Or, provide a load resistor from V+ (relay power) to
the top of the interlock chain to increase the available current (bypassing
the relay coil).

Here is a suggested circuit which will discharge the high value main filter
capacitors quickly and safely can be permanently substituted for the normal
bleeder resistors. A visual indication of charge is provided from maximum
input to a few volts. The dual polarity version of this circuit is described
in the document:
Capacitor Testing,
Safe Discharging and Other Related Information and can be built into a
handy capacitor discharge tool.